It is known to produce lightweight insulating boards from an aqueous suspension of hydraulically setting binders containing calcium silicate, a reinforcing fiber material and fillers which harden either in air, under normal conditions, or under steam pressure. In said procedure it is necessary that the components of the binder, namely calcium silicates or hydrated lime, react with silica and water to form the known calcium silicate hydrate structures. The structure of the set binder will depend on the relative proportions of hydrated lime and silica which are capable of actually reacting under the reaction conditions. The properties of the product further depend, to a significant extent, on the reinforcing fiber material which is added into the aqueous suspension. Asbestos had proven to be useful for this purpose. However, it has recently become necessary to abstain from using asbestos fibers for reasons of environmental protection.
In the meantime, a great many so-called asbestos-free products have been developed and marketed. However, these products generally have significant drawbacks over the asbestos-containing products, since certain desired properties can only be achieved at the expense of other desired properties. This, accordingly, limits the general usefulness of the asbestos-free products, as compared to that of the asbestos-containing products.
One typical field of application for asbestos fibers is lightweight insulating boards, more specifically, those having high fire-resistance. It has been shown that lightweight insulating boards can be manufactured by replacing the asbestos fibers with cellulose fibers and/or organic fibers and/or alkali-resistant glass fibers. However, these products generally have inferior mechanical properties and are less flame-resistant. They further fail to meet the requirements of the toxicity test according to DIN 53436, since poisonous gases may be formed therefrom at high temperatures.
As a filler for flame-resistant insulating boards, mica has proven valuable; mica, if desired, has been employed together with light fillers, expanded perlites or puffed clay; cf. the British Patent Specification No. 1,498,966. In said Patent Specification there have been mentioned, as binders, hydraulically setting calcium silicate binders, Portland cement, alumina cement and blast furnace slag cement. Thus, the binders used are hydraulic binders which will only set under autoclave conditions or binders setting with water at normal temperature.
It is known that binders setting at room temperature have mechanically inferior properties, more specifically, a poorer flexural strength, and an inferior water resistance than the binders having been hardened in an autoclave.
Thus, in practice asbestos-free, flame-resistant lightweight insulating boards were prepared from products hardened in an autoclave in which, more particularly, tobermorite structures have been formed. Thus, these crystalline calcium silicate structures so far have been considered the indispensable prerequisite to obtain products which are thermally stable as well as capable of sustaining mechanical load.
However, the required setting in an autoclave exhibits considerable disadvantages:
1. Hardening in an autoclave is very energy-consuming and may even cost more than 10% of the employed raw material cost. For the reinforcing fiber material, only cellulose and a few very expensive organic fibers can be used, as under the autoclave temperatures and alkaline conditions, glass fibers, as well as most organic fibers, are destroyed.
2. Under the conditions of autoclaving, the crystalline tobermorite structure can be formed only if free crystalline silicon dioxide, e.g. quartz, is present. Even under ideal conditions, a certain amount of unreacted silicon dioxide remains in the final product. However, since quartz dust is dangerous to human health, official regulations require that only minimum amounts of crystalline quartz be present in the final product. However, when other silicon dioxide sources, e.g. amorphous silica, fly ash etc., are used, the thermal stability of the final products at high temperatures significantly decreases.